The Effect of Temperature and Concentration on the Rate
Evaluation Like any experiment, there were a number of potential errors during the procedure of the experiment. Errors could have arisen as a result of the uncertainties associated with the instruments I used to take measurements, and also as a result of errors associated with the actual method. Of course, due to the limitations of the procedure, they could not be eliminated completely, so I will explain what I did to reduce them to an acceptable level and how I could have improved my method to reduce them even further.
The following table shows the reasons for my choice of equipment in carrying out my method. Equipment| Justification| 100 cm3 burette| I needed to accurately measure out large quantities of hydrogen peroxide (90 cm3 and 150 cm3). The 100 cm3 burette is a precise instrument and would allow me to measure out the hydrogen peroxide by filling it fewer times than I would need to with 50 cm3 burette. | 50 cm3 burette| I needed to repeatedly measure out small volumes of solutions A–I. The burette made the task convenient, and it is a precise instrument. 250 cm3 volumetric flask| I needed to make up a specific volume of a standard solution. The volumetric flask has a low error. | 100 cm3 volumetric flask| I needed to make up a specific volume of a standard solution. The volumetric flask has a low error. | Top pan balance| I needed to accurately weigh out small amounts of solid when making up my solutions. | 25 cm3 Mohr pipette| I used the pipette to accurately transfer sulfuric acid when making up solutions. I could not do this with a volumetric pipette, as the volume I transferred was 20 cm3. Distilled water| I used the distilled water to wash out any glassware and storage jars before using them to avoid contamination. | Crushed ice| I used the ice to cool my reactants down to 10 °C. | Water bath| I used the water bath to heat my reactants up to 30 °C, 40 °C and 50 °C. It kept the temperature constant—it does not cool down like hot water in a beaker. | Thermometer| I needed to measure the temperature of the reactants before pouring them into the beaker and stirring them. | Magnetic stirrer| I used the stirrer to ensure the reaction mixture was uniformly mixed.
This was necessary to produce sharp colour changes. | Stopwatch| I used the stopwatch to record the times of the colour changes. These are the values I needed to investigate the effect of temperature and concentration on rate. | Measurement errors These are the errors associated with the equipment I used when weighing out solids, measuring volumes of liquid, recording the temperature of my reactants, and recording the times of the colour changes. Equipment| Error| 100 cm3 burette| ±0. 2 cm3| 50 cm3 burette| ±0. 1 cm3| 250 cm3 volumetric flask| ±0. 3 cm3| 00 cm3 volumetric flask| ±0. 2 cm3| 25 cm3 Mohr pipette| ±0. 1 cm3| Top pan balance| ±0. 005 g| Thermometer| ±0. 5 °C| Stopwatch| ±0. 005 s (for instrument), ±0. 5 s (for measurements), ±0. 05 s (for measurements at 50 °C)| The stopwatch could record to 2 d. p. but the times I recorded were affected by my reaction time. Recording to 2 d. p. would be pointless, as I could not record that precisely. I decided to record the times to the nearest second, except for my results at 50 °C, where I recorded them to 1 d. p. because of the short duration of time between the colour changes.
Percentage uncertainties Using the measurement errors, I can work out the percentage uncertainties for my measurements. I can do this using the formula: percentage uncertainty = error / value of measurement x 100% I made multiple measurements with many of the instruments I used. For these measurements, I will find the uncertainties for three of the values (the highest, the lowest and one close to the average) to give an indication of how the uncertainty changed across the range of measurements I made. Equipment| Error| Measurement| Percentage uncertainty / %| 100 cm3 burette| ±0. cm3| 150 cm3| (I used the burette twice, so 0. 4 / 150 =) 0. 27| | | 90 cm3| 0. 22| 50 cm3 burette| ±0. 1 cm3| 10. 00 cm3| 1. 0| | | 5. 00 cm3| 2. 0| | | 1. 00 cm3| 10| 250 cm3 volumetric flask| ±0. 3 cm3| 250 cm3| 0. 12| 100 cm3 volumetric flask| ±0. 2 cm3| 100 cm3| 0. 20| 25 cm3 Mohr pipette| ±0. 1 cm3| 20 cm3| 0. 50| Top pan balance| ±0. 005 g| 26. 75 g| 0. 02| | | 10. 7 g| 0. 05| | | 0. 85 g| 0. 59| Thermometer| ±0. 5 °C| 50 °C| 1. 0| | | 30 °C| 1. 7| | | 10 °C| 5. 0| Stopwatch| ±0. 5 s| 437 s| 0. 11| | | 95 s| 0. 53| | | 1 s| 50| | ±0. 05 s (at 50 °C)| 31. 8 s| 0. 57| | | 12. 9 s| 0. 388| | | 1. 5 s| 3. 3| The percentage uncertainties varied wildly depending on the error of the instrument and the value of the measurement. The largest uncertainty (50%) came from the stopwatch when I used it to record a time of 1 s. However, this would not have affected my calculations to a great extent, as I only used the time to calculate the blue cycle for the first oscillation. It would not have affected the value I calculated for the average oscillation period by a significant amount, and would not have noticeably affected the trends in my graphs.
This applies to all uncertainties from the stopwatch. I could have recorded all my times to 1 d. p. to improve the accuracy of my calculations and draw graphs that showed a trend closer to the true one. The second most significant uncertainty (10%) was for the burette when I used it to add 1 cm3 of solution to different test tubes in order to test the effect of changing the concentration of propanedioic acid, manganese(II) sulfate(VI) and sulfuric acid. This is a very significant error that could have definitely weakened the accuracy of my results.
It might explain, for instance, the wildly varying number of oscillations I observed for tests at 0. 01 M manganese(II) sulfate(VI), as well as the increased appearance of anomalous results at lower concentrations. Even the uncertainty for a measurement of 10 cm3 using the burette was 1%, which is significant. In order to reduce the instrumental error, I could have used a 1 cm3 pipette or syringe to measure very small volumes of solution. I could not have done much more to conveniently transfer larger volumes of solution (i. e. p to 10 cm3) while reducing the error, as even a 10 cm3 pipette has the same error as a 50 cm3 burette, and it would have been extremely time-consuming to transfer my solutions to test tubes using a 1 cm3 pipette. Still, a 1% uncertainty would not have dramatically affected my results. Another source of significant percentage uncertainties was the thermometer—at every temperature the uncertainty was above 1%. At 10 °C, it was 5%, which is particularly significant. This means that I could have started stirring the reactants at a temperature between 9. 5 °C and 10. 5 °C.
However, there were no thermometers more precise than ±0. 5 °C, so there is not much I could have done to reduce this error. Anyway, looking back at my raw results, the times I recorded for tests at 10 °C were not particularly discordant in comparison with the results I obtained for the other temperatures. All other errors were below 1%, so were insignificant. I used the volumetric flasks correctly, using a Pasteur pipette to add the distilled water for the last centimetre below the graduation mark, checking the mark at eye level in order to make sure I stopped at the correct point.
I took readings from the bottom of the meniscus at eye level when using the Mohr pipette and burettes to reduce parallax error. I had to round up the mass of manganese(II) sulfate(VI)-1-water I weighed on the top pan balance from 0. 845 g to 0. 85, so an 4 d. p. analytical balance would have been better for this, but I did not have access to one. Procedural errors These are the errors that could have arisen from the method and improper technique. When making up solutions, it is important to rinse out the glassware and other equipment with distilled water before use.
This was particularly vital for the BR reaction, due to its high sensitivity to chloride ions. As mentioned in my method, I did wash out all equipment with some distilled water before putting them in contact with any reactants to minimise the risk of contamination. It would have been impossible to prevent a small amount of solution from being lost when transferring them. When transferring from a beaker through a funnel to volumetric flask, the small amount left would have led to a lower final concentration then planned. I minimised this error by washing out the beaker with distilled water three times.
When pouring solution from the test tubes into the reaction beaker, a small amount is also lost. However, the amount left would have little effect on the results because it is a systematic error, i. e. it is repeated every time the solution is poured. I always inverted the volumetric flasks when making up solutions in order to ensure homogeneity. Before pouring them into the burettes, I gave the storage bottles a swirl in case the uniformity of the solution had been affected during storage. This would prevent the trials from being tested at different concentrations, which would have compromised the accuracy of my results.
In addition, I used a magnetic stirrer to make sure the consistency of the solution remained even within the reaction beaker. This also meant that the colour changes were sharper. It was especially important that the blue colour change was sharp, as this is the value I used to calculate the oscillation period, and therefore, rate of reaction. However, because human reaction time is not perfect, there was always some delay between the colour change and the pressing of the stopwatch. This is why I could not record times accurate to 2 d. p.
At higher temperatures, i. e. 40 °C and 50 °C, the water from the solutions in the test tubes evaporated a lot faster than at room temperature while being heated in water bath, which would have increased the concentrations of the reactants and overstated the effect of the temperature increase. I minimised this error by removing the test tubes from the water bath as soon as possible after the temperature of the reactants reached the appropriate level. Next time, I would seal the test tubes using stoppers to prevent any water vapour from escaping.
Unfortunately, the reactants could not remain at their starting temperature while being stirred, as they had to be poured into a beaker and set on a magnetic stirrer. This means that during tests at 30 °C, 40 °C and 50 °C, the reactants cooled down; at 10 °C, the reactants warmed up. This would have understated the effect of temperature on rate. There was a problem with the hydrogen peroxide in the burette. Because it was stored in the fridge, it was cold when I took it out. As it warmed up, there were noticeable increases in the level of solution in the burette.
Trials that were run near the start of the session may have used colder, more concentrated hydrogen peroxide, which would have affected the rate of reaction. I only took the temperature of reactants when I tested the effect of temperature. In order to resolve this problem next time, I would take out the hydrogen peroxide at the very start of the lesson and wait for it to warm up while setting up the other burettes, magnetic stirrer etc. and also take the temperature of the reactants when testing concentration to see if it might have had a secondary effect on the rate.
The potassium iodate(V) was not soluble enough to make Solution F (potassium iodate(V), 0. 5 M). Although I did manage to fully dissolve it with the aid of heat, a small amount crystallised out of solution after it cooled down, which would have decreased the solution’s concentration and affected the results I obtained for the tests where I changed the concentration of potassium iodate(V) and sulfuric acid. Next time, I would change the experiment and run the tests at lower concentrations. Reliability My results were quite reliable, as I ran the reaction three times at each temperature and concentration.
The number of oscillations was usually the same at each temperature/concentration and the times were concordant to an acceptable degree. There were a few anomalous runs, which I mentioned in my analysis section, and gave a possible explanation for above. I could have repeated the experiment a further time when I got inconsistent results, e. g. 0. 01 M manganese(II) sulfate(VI), to increase reliability. Extending the investigation The observations I made about the colours during particular runs were solely qualitative.
I could broaden the scope of my investigation by using colourimetry to obtain a quantitative measurement of the colour intensity when the reaction was especially faint or dark. I could then compare it to values from the standard reaction to reinforce my observations. I could also use the data logger to measure the times of the colour changes. I could then compare the results from this technique to those from the stopwatch and evaluate the advantages and disadvantages to both methods, and decide which one would be better at producing accurate results.
Conclusion Overall, I am satisfied that I have made valid conclusions about the effect of temperature and concentration on the rate of the Briggs–Rauscher reaction. Although I did not fully meet my aim of finding the order of reaction for every reactant, I did discover that the reaction was not typical in this sense, and that the orders of reaction could not easily be found. I did manage to justify parts of the mechanism through the qualitative observations I made.